1. Field of the Invention
The present invention relates to solid, superacid catalysts, based on zirconia and mixed sulfated oxides, obtained directly via sol-gel, with a non-alkoxide synthesis method, in the forms of use, microspheres, or spheres having dimensions of up to 3 mm and over, and with textural and mechanical characteristics which make them suitable for isomerization processes of hydrocarbons, verified by means of the model conversion reaction of n-butane to iso-butane.
2. Description of the Background
Interest in solid (super)acid catalysts derives from considerations of an environmental nature, associated with improvements in industrial isomerization and alkylation processes of hydrocarbons, which use large quantities of sulfuric or hydrofluoric acid in emulsion.
If solid acids can be compared with the known liquid superacids, the question will remain open until a clear definition of solid superacid is formulated [J. Sommer et al., Catal. Today, 38 (1997)309]. In fact, whereas the definition of liquid superacid is based on the definition of R. J. Gillespie [Adv. Phys. Org. Chem., 9 (1972)1]: any acid stronger than sulfuric acid at 100% (H.sub.0 =-12 in Hammett's acidity scale), the most convincing evidence of the superacid nature of a solid should be, viceversa, its capacity to reversibly protonize an alkane, exploiting the .sigma.- basicity of the C--H or C--C bond [G. A. Olah, Angew. Chem. Int. Ed. Engl., 12 (1973) and J. Sommer et al., J. Am. Chem. Soc., 19 (1997)3274].
Among all linear alkanes, n-butane is the most difficult to isomerize, requiring the formation of a primary isobutyl cation. On the other hand, isobutane is an essential intermediate for the production of alkylated and oxygenated products for fuel.
It has been known for some time that sulfated zirconia is a solid acid, active in the isomerization of n-butane [M. Hino et al., Chem. Lett., (1979)1259]. The tetragonal, rather than monoclinic sulfated phase, seems to be the active crystalline phase of this material [C. Monterra et al., J. Catal., 157 (1995) 109].
The doping of sulfated zirconia with transition elements favourably influences the isomerization rate of n-butane, in particular the combination of mixed oxides of iron and manganese [F. C. Lange et al., Catal. Lett., 41 (1996)95]. The addition of 0.4% Pt to a sulfated zirconia, doped with iron and manganese oxides, produces an increase in the activity with respect to sulfated zirconia alone [X. M. Song et al., Catal. Lett., 37 (1996)187]. The role of the metal consists in favouring the hydrogenation/dehydrogenation mechanism, by inhibiting the formation of carbonaceous substances, which can deactivate the system. Its role therefore consists in establishing a bifunctional mechanism, by increasing the surface concentration of the olefins, rather than increasing the acid strength of the sites [F. Garin et al., J. Catal., 151 (1995)26]. As a consequence, a considerable increase in the life of the catalyst is observed [J. C. Yori et al., Appl. Catal., 129 (1995)83] and, following this scheme, doping with iridium and platinum allows more active sulfated zirconias to be obtained [M. Hino et al., Catal. Lett. 30 (1995)25 and J. C. Yori et al., Appl. Catal.:A, 129 (1995)83]. A promoting role of aluminum has recently been observed, which is different from that of transition metals, in stabilizing the surface sulfated complex and increasing the number of acid sites with intermediate acid strength, the most effective for the isomerization of n-butane [Z. Gao et al., Topics in Catalysis, 6 (1998)101].
Finally, WO.sub.3 also makes zirconia a strong solid acid [M. Hino et al., Chem. Commun., (1987)1259] and Pt (0.3%) promotes the selective hydroisomerization of alkanes, from n-heptane upwards, at 373-473K, without the formation of carbonaceous substances [E. Inglesia et al., J. Catal., 144 (1993)238].
U.S. Pat. No. 5,750,459 of the same applicant describes a sol-gel process for the production of spheres and microspheres of pure zirconia or mixed oxides based on zirconia, obtained starting from a basic zirconium carbonate, and useful as catalysts or carriers for catalysts. This patent provides a detailed description of the procedure for obtaining pure zirconia spheres, example 1, pure zirconia microspheres, example 2, and spheres of mixed zirconia-alumina oxides (10% by weight), example 4.
PCT/US97/07424 describes a sol-gel process for the production of sulfated zirconia, obtained starting from zirconium alkoxides mixed with acetylacetone in a solution of ethanol, and precipitated in an acidified mixture of a C.sub.8 -C.sub.18 alkylamine and water. The sulfation process with diluted sulfuric acid takes place on powders dried at 110.degree. C., after precipitation, aging, centrifugation and extraction with ethanol. The end-catalyst, sulfated zirconia in powder form, is obtained after calcination of the solid at temperatures ranging from 600 to 750.degree. C. for about two hours. As an alternative, the same patent describes a process in which sulfated zirconia in powder form is obtained by hydrolysis of a solution of zirconyl chloride in diluted ammonia, washing with water and drying at 110.degree. C., powdering and impregnation in diluted sulfuric acid for 5 hours, filtration and calcination at about 650.degree. C.
CA-2069373 describes a process in which the life of the superacid catalyst is prolonged if the isomerization is carried out under supercritical conditions or almost critical conditions. Example 1 of this patent describes a procedure in which the superacid catalyst in powder form is obtained by hydrolyzing at pH 7-8, with an ammonia solution, a solution of a zirconium salt, obtained starting from a zirconium carbonate dissolved in nitric acid. The precipitate is separated, washed, filtered, dried and ground. It is then impregnated using the "incipient wetness" method with a solution of ammonium sulfate, dried and calcined at 725.degree. C. The sulfated zirconia thus obtained contains a percentage of sulfates of 4%. Optionally, the example also describes the incorporation of one or more transition metals, for example Fe and Mn, onto the sulfated zirconia. The catalyst is obtained, alternatively, by impregnating a salt of the metal either onto dried zirconium hydroxide or sulfated hydroxide.
A similar superacid catalyst, based on sulfated zirconia, obtained however starting from a solution of zirconium oxychloride, hydrolyzed with a solution of ammonia, and then impregnating the dry powder with sulfuric acid, and others based on sulfated titania and sulfated iron oxide are described in U.S. Pat. No. 5,017,699. These catalysts are claimed in the synthesis of polyalkyl ethers of polymethylolmelamine compounds.
EP-0759423 describes a superacid catalyst obtained, again starting from a solution of zirconium oxychloride, as in the previous patent, but impregnating the dry product with a solution of ammonium sulfate and calcinating at 650.degree. C. This catalyst is useful in the synthesis of isobornyl methacrylate or acrylate.
JP-01/245853 describes a process for obtaining a catalyst with a high alkylation activity. Metals of group IIb, such as Zn, Cd, or of group Va, such as V, or of group VIa, such as Cr, Mo or of group VIIa, such as Mn, or their compounds, and a radical of sulfuric acid or one of its precursors, are supported on a hydroxide or oxide of metals of group III, such as Al, Ga and/or of group IV, such as Ti and Zr, followed by calcination and stabilization at temperatures of about 400-800.degree. C.
EP-0653397 discloses a method for the preparation of superacids based on sulfated zirconia, which incorporates, by means of "incipient wetness" impregnation, heteropolyacid (HPA) components or polyoxoanionic (POA) components, which give advantageous properties for effecting alkylation processes in liquid phase of isoparaffins to olefins. In the invention, preference is given to HPA or POA having the Keggin structure, represented by the formula H.sub.4 XM.sub.14 O.sub.40, wherein X can be any metal of groups IV, V, VI, VIII, or lanthanides, and M is any element of groups III, IV, V, VI.
EP-0661254 describes a process for improving the oxidative dehydrogenation of light alkanes to olefins, or of alkylaromatic compounds, using a solid superacid catalyst. Examples of solid superacid catalysts are: sulfated zirconia, sulfated titania, sulfated alumina, halogenated alumina, etc.; used alone or combined with one or more metals capable of increasing the acidity, preferably V, Cr, Mn, Fe, Zn, Co, Sn, Pb, Ca and Sb.
WO-98/09727 describes a superacid catalyst formed with traditional ceramic techniques.
EP-0504741 discloses a method for producing hydrogen peroxide directly in a reaction medium containing a promoter, for example, a halogenated compound, in the presence of a metal of the platinum group supported on a solid acid or solid superacid. The solid superacid consists of sulfated zirconia, alumina or titania, calcined preferably from 400 to 600.degree. C. As there may be problems of dissolution of the sulfate ion if the superacid is placed for long periods of time in a reducing environment, a solid superacid consisting of metal oxides is preferable for this type of reaction. These metal oxides preferably consist of molybdenum oxides or tungsten oxide supported on zirconia. The calcination temperatures preferably range from 600 to 800.degree. C. The form of solid superacid can be a microfine powder, grains or pellets. No mention is made in this patent of the techniques, preferably of the traditional ceramic type, for obtaining these forms.
Finally, JP-06/321878 describes in an example a solid superacid obtained by impregnating zirconium hydroxide with 8 parts by weight of sulfuric acid 1N and, after drying, calcination at 570.degree. C. for 3 hours. The catalyst is claimed as being useful in the synthesis of esters containing amine groups.
None of the above processes describes a reliable method for the production, via sol-gel, of solid superacid catalysts based on zirconia, directly into a form which can be used in industrial isomerization and alkylation processes of hydrocarbons with a low molecular weight.
As previously specified in the known art, processes using synthesis phases possibly belonging to sol-gel methods, although more similar to precipitation techniques, invariably lead to the production of more or less agglomerated fine powders (which therefore also need grinding phases), from which the synthesis of the solid superacid is initiated.
Bearing in mind that interest in solid superacids is due to the possible substitution of traditional strong liquid acids, such as H.sub.2 SO.sub.4 or HF, for considerations of an environmental nature and to comply with regulations which are becoming stricter and stricter in this field, it is evident however that a solid superacid cannot be used directly in the form of a very fine powder. In fact, this would imply facing very serious problems of liquid-solid separation which are not easy to solve.
The only alternative is to resort to the traditional pelletization, extrusion or pressing, ceramic techniques as mentioned for example in WO-98/09727 and EP-0504741.
These techniques inevitably have counter-indications due to heterogeneity in the structure of the end-catalyst and therefore in its textural properties and also due to the use of various additives, both organic and inorganic, whose influence on the catalytic properties of the system is often not known or ignored.
Sol-gel techniques are absolutely preferable from the point of view of homogeneity of the chemical compositions; however neither techniques which use costly metal alkoxides, in particular zirconium, as starting materials, for example in PCT/US97/07424, nor those using less costly salts, for example in CA-2069373, offer solutions for the direct synthesis of a form of solid superacid catalyst which can be used under real isomerization process conditions.
U.S. Pat. No. 5,750,459 discloses a sol-gel process for the production of spheres (and microspheres) of pure zirconia or mixed oxides based on zirconia, useful as catalysts or carriers for catalysts. However it does not disclose any process which can be simply used by any expert in the field for obtaining solid superacids based on zirconia.
In fact the most widely-used impregnation technique, the "incipient wetness" method, which can alternatively be used on dried material or calcined material, does not provide any useful result in this specific case. Experiments effected by impregnating with solutions of sulfuric acid or ammonium sulfate, more or less dilute, have not produced at the end of the calcination process at the temperatures indicated in literature of 400-700.degree. C., structurally resistant spheres but pretensioned materials susceptible to considerable cracking, before and mainly after catalytic isomerization tests. The phenomenon is more serious when Pt, for example, is used as promoter.
A solution could therefore be to introduce the precursor of sulfate ions directly into the sol and follow the procedures for the preparation of spheres as described in the examples of U.S. Pat. No. 5,750,459. However, apart from the integrity of the spheres at the end of the calcination treatment, there would be no control over the stoichiometric ratio Zr.sup.4+ /SO.sub.4.sup.2- as, after dripping the sol into the gelation bath, during the aging of the spheres of gel in the bath, dissolution kinetics of the sulfates present inside the sphere would be established, which would depend on a considerable number of parameters and in particular the aging time and relative gel/gelation bath volumetric ratio.